This invention relates generally to drilling. More specifically the invention relates to drilling holes in earthen formations.
Existing methods known in the art for drilling in earthen formations or other non-homogenous and relatively hard mediums may encounter several difficulties, especially at deeper depths using extended drill strings and bottom hole assemblies. One potential concern, backward whirling, may be encountered when an imbalanced rotation or lateral movement of the bottom hole assembly causes impact, even briefly, with the borehole wall, or possibly a stabilizer location.
Backward whirling may occur when the borehole wall, or other object, impacts or is impacted by a spinning drill string. When the spinning bottom hole assembly contacts the borehole wall, the point of contact on the bottom hole assembly may be urged to rotate in a direction opposite the rotational direction of the bottom hole assembly. As drilling speed (“RPMdrill”) increases, backward whirling speed (“RPMwhirl”) can increase dramatically, especially as the difference between the borehole diameter (“Dborehole”) and the bottom hole assembly (“DBHA”) decreases per the equation:
      RPM    whirl    =            -                        D          BHA                                      D            borehole                    -                      D            BHA                                ⁢          RPM      drill      This may not only result in lost energy and slower overall rotation of the bottom hole assembly, but damage thereto, possibly necessitating time consuming fishing-out operations.
Additionally, lateral bending of the drillstring and/or bottom hole assembly may occur due to reaction of the drill string and/or bottom hole assembly to the impact. As backward whirling increases, more frequent, larger amplitude shocks may occur to the drilling system. This may cause loss of drilling speed, related system damage (for example, a rotary table or rotational drive damage), as well as drill bit damage from unexpected lateral motion transmitted down the drill string.